This is one of my all time favorite quotes, but I had always heard it attributed to another great writer with a talent for the insult: Mark Twain. It's included in "The Wit and Wisdom of Mark Twain":
http://books.google.com/books?id=Ee_...0twain&f=false
but I don't know if it can actually be found in his letters or any other actual documents of his, or what the original source might be...